Silver halide emulsions are inherently sensitive to blue and ultraviolet radiation, while they are not sensitive to the green and red radiation. For normal color photographic processes, it is necessary to make these emulsions sensitive to green and red radiation by means of spectral sensitizers (e.g. cyanine dyes), well-known to the man skilled in the art.
It is also known that ultraviolet (UV) radiation may damage the color balance of the color photographic images (intended as the capacity of such images to reproduce real images with the same color balance as seen by human eye), because UV radiation causes an exposure, and therefore color formation, without any correspondence to the real image, as seen by human eyes, which do not see UV radiation, i.e. those shorter than about 420 nm.
Such radiation, furthermore, has destroying effects on the elements which form the photographic image when they are exposed to the light after having been processed (for instance color paper, after exposure and processing, undergoes a color degradation if it does not contain a suitable ultraviolet absorber).
Some compounds, such as hydroxy-phenylbenzotriazole compounds, capable of protecting photographic materials from destroying effects of UV radiations, have been described in U.S. Pat. Nos. 3,004,896; 3,253,921; 4,323,633. Such compounds have found large use in photography, particularly in color photography.
These compounds cannot be used, however, to prevent UV radiation from disturbing the chromatic equilibrium of photographic images.
To accomplish this result, compounds must absorb the ultraviolet radiation near 400 nm and not that near 420 nm, while the above mentioned hydroxy-phenylbenzotriazole compounds absorb between 300 and 370 nm.
Even if some compounds with such absorption characteristics were known, they can loose those characteristics when introduced into the photographic layers by the normal techniques known in the art, such as the dispersion technique. An example of this is represented by the 3-dialkylaminoallylidenemalononitrile compounds described in U.S. Pat. No. 30,303 wherein the two alkyl chains, being the same or different, each contain 1 to 10 carbon atoms, which have shown to be compounds with a high and sharp absorption near 400 nm (and a high molar extinction coefficient), without absorbing the radiations near 420 nm. Unfortunately, they have been shown to loose their characteristics when introduced in the photographic layer according to the above mentioned dispersion method. In fact, when the alkyl chains are short (1 to 3 carbon atoms), they tend to crystallize or to diffuse into the layers; when the alkyl chains are long (at least 4 carbon atoms) they exhibit an undesired absorption at 415 nm. To obviate this disadvantage, the loaded polymer technique has been suggested (see BE patent 833,512) which consists of loading solid particles of a particular polymeric latex with an aminoallylidenemalononitrile hydrophobic derivative and mixing the so loaded latex with the gelatin of the photographic layer, in which the UV absorber compound must be introduced. This technique, however, has the disadvantage that it is not suitable for obtaining consistent, reproducible results. Besides, not every polymeric latex is suitable with this process and those that are suitable are difficult to prepare and expensive to purchase. Furthermore, the high latex/UV absorber compound ratio makes necessary the use of quantities of latex which are too high and negatively affect the physical characteristics of the layer containing it.
Polymeric compounds including ultraviolet absorbing aminoallylidene units, obtained upon copolymerization of 3-aminoallylidenemalononitrile with an ethylenically unsaturated monomer (such as an acrylic monomer), have been described in U.S. Pat. No. 4,307,184, but this technique seems more complicated than the normal dispersion techniques known in the art (see for example U.S. Pat. Nos. 2,322,027; 2,533,514; 2,801,171; 2,870,012; 2,991,177; 2,739,888; 3,253,921 and in British patent 1,357,372).
Such solvent dispersion technique consists of dissolving a compound in an organic solvent and then dispersing the obtained solution with an aqueous medium, such as water or a gelatin in water solution. The obtained dispersion can either be directly introduced into the photographic composition before coating or can be dried to remove part or all organic solvent prior to such introduction. In one case, high-boiling (water immiscible) organic solvents are to be used. In the other case, low-boiling organic solvents are to be used alone or mixed with high-boiling organic solvents.